The Descent of Man

"Homo sapiens did not appear on the earth, just a geologic
second ago, because evolutionary theory predicts such an outcome based on
themes of progress and increasing neural complexity. Humans arose, rather,
as a fortuitous and contingent outcome of thousands of linked events, any
one of which could have occurred differently and sent history on an
alternative pathway that would not have led to consciousness. To cite just
four among a multitude:

If our inconspicuous and fragile lineage had not been among the few
survivors of the initial radiation of multicellular animal life in the
Cambrian explosion 530 million years ago, then no vertebrates would
have inhabited the earth at all.

If a small and unpromising group of lobe-finned fishes had not
evolved fin bones with a strong central axis capable of bearing weight
on land, then vertebrates might never have become terrestrial.

If a large extraterrestrial body had not struck the earth 65 million
years ago, then dinosaurs would still be dominant and mammals
insignificant (the situation that had prevailed for 100 million years
previously).

If a small lineage of primates had not evolved upright posture for
the drying African savannas just two to four million years ago, then
our ancestry might have ended in a line of apes that, like the
chimpanzee and gorilla today, would have become ecologically marginal
and probably doomed to extinction despite their remarkable behavioral
complexity. . .

Our conventional desire to view history as progressive, and to see
humans as predictably dominant, has grossly distorted our interpretation
of life's pathway by falsely placing in the center of things a relatively
minor phenomenon (humans) that arises only as a side consequence."
(Gould, 1994).

In this lecture we will trace the evolution of humanity beginning with
the origin of primates through the comings and goings of Genus Homo.

Below is a trace through time of the origin of humans including various
key events. Time is in units of millions of years before present. Note
that these dates are approximations at best. The only consistent thing I
can say about the various dates for various events is that those supplied
by various authors are not consistent. For example, one author places the
tree shrew-like progenitor of primates as appearing 55 million years ago
rather than the 80 million years ago indicated below. Thus, other than the
various items listed as occurring 65 million years ago (as well
established a date as any), don't assume that the absolute dates (or even
approximations) are cast in easily dated stone. However, relative dates
often are on much firmer ground. In other words, should you decide to
recall the year that orangutans diverged from the rest of Hominoidea,
think of it as about eight million years ago assuming that gibbons
diverged about 10 million years ago, etc. Such is the nature of our
understanding of the deep past.

*Binocular vision means having eyes placed in the front of heads
with overlapping fields of vision. This arrangement allows the brain to
interpret the visual world much more efficiently as three dimensional
space.

Diet driven evolution:

Primates began as insect eaters.

With time, however, primates came to rely on diets based to a
large extent on the vegetation found in the canopies of trees.

"Selective pressures . . . favored considerable enhancement of
the visual apparatus (including depth perception, sharpened acuity and
color vision), thereby helping primates travel rapidly through the
three-dimensional space of the forest canopy and easily discern the
presence of ripe fruits or tiny, young leaves. And such pressures
favored increased behavioral flexibility as well as the ability to learn
and remember the identity and locations of edible plant parts. Foraging
benefits conferred by the enhancement of visual and cognitive skills, in
turn, promoted development of an unusually large brain, a characteristic
of primates since their inception. . . As a group . . . primates have
depended most strongly on selective feeding and on having the brain
size, and thus the wit, to carry off this strategy. Other plant-eating
orders, in contrast, have tended to focus heavily on morphological
adaptations. . . Primates typically have larger brains than do other
mammals of their size. (The author) believes the difference arose
because primates feed very selectively, favoring the highest-quality
plant parts." (p. 86, 89, 90, Milton, 1993)

Pongidae is a now largely discredited grouping of the great apes
separately from that of human lineage.

Paraphyletic taxon:

Based on anatomic considerations alone, the grouping Pongidae
was once considered to be monophyletic.

Recent molecular data strongly argue that instead it is paraphyletic.
That is, humans are more closely related to both chimpanzees and
gorillas than any are to orangutans (i.e., the last common ancestor of
humans, chimpanzees, and gorillas for a few million years coexisted with
and was not identical to the ancestral orangutan).

Note that even a grouping of chimpanzees and gorillas would now be
considered paraphyletic since humans and chimpanzees are considered to
be more closely related than chimpanzees and gorillas.

Here we will employ Hominidae to describe the apes and consider Pongidae
to be a subset of Hominidae, and one which is a valid clade only so long
as humans are considered also to be members of Pongidae.

The orangutan and the rest of Hominoidea last shared an
ancestor prior to the divergence of gorillas, chimpanzees, and hominids
from each other.

That is, at the time of divergence of the orangutan, Hominoidea
consisted of three distinct and one less distinct group:

those direct ancestors of orangutan

the various ancestral gibbons and gibbon-like lineages

the single common gorilla, chimpanzee, hominid ancestor

all the various other lineages that became extinct prior to modern
times.

In particular, "At the molecular level, gorilla, chimpanzee, and
human beings are more closely related to one another than is any one of
these to the Asian great ape, the orangutan." (p. 144, Tobias,
1992)

Members of family Hominidae include all of the upright walking
(bipedal) apes, extant and extinct.

Note that the oldest members of family Hominidae is considered
to have diverged from the common human-chimpanzee ancestor.
Consequently, family Hominidae can simultaneously be considered
to be monophyletic and to exclude all of the chimpanzees.

Uncertainty:

The timing of the divergence between family Hominidae and
chimpanzees is best estimated from molecular data.

Unfortunately, from molecular data it is difficult to infer
morphological (much less behavioral) information.

This, combined with a fragmentary fossil record from the time of this
split results in investigators having little knowledge of the appearance
of the human (and chimpanzee) ancestors immediately prior to and
following their divergence.

That is, it is thought the first members post-divergence from the
chimpanzee lineage were likely not bipedal (with bipedalism developing
only later), though we don't yet the have strong evidence that this
actually is true.

What we do know is that, minimally (and ultimately), hominids came to
consist of two evolutionary branches, one consisting of genus Homo
and the other consisting genus Australopithecus.

Since we know little about the last common ancestor of humans and
chimpanzees, as well as the immediately post-common ancestor of humans, we
cannot say with reasonable conviction that the earliest, post-common
chimpanzee linaege ancestors of family Hominidae walked upright, though
clearly this was a prominent feature in future hominid evolution.

Evolution from chimp-like morphology:

Given the relative lack of divergence of chimpanzees from gorillas,
compared with that exhibited by humans, it is more probable that the
last common ancestor to both humans and chimpanzees looked and acted
more like a chimpanzee than like a human.

"The spread of lighter woodland and savannah, and the retreat
of the margins of the primeval forests, could well have created
conditions in which the tendency toward an erect posture and
bipedalism was favored. The ability to run across the high grass cover
of the savannah, perhaps from one woodland stream to another, might
have held advantages for those apes that could 'walk tall,' since
uprightness would have enabled its possessors to see over the high
grass and to watch out for predators like lions and saber-toothed big
cats." (p. 149, Tobias, 1992)

The first members of family Hominidae appear not to have
had relatively large brains (i.e., relative to that of chimpanzees).

Particularly, there is strong fossil evidence that later members of
family Hominidae simultaneously possessed both bipedalism
and small brains (i.e., chimpanzee size).

Since chimpanzees do not walk upright (and, indeed, employ a method of
locomotion similar to that employed by gorillas), in all likelihood the
earliest, non-chimpanzee lineage, common ancestor of Hominidae
neither walked upright nor had a large brain.

The origin of family Hominidae is routinely considered to be
traceable to Eastern Africa.

Prior to the chimpanzee-Hominid split, all of the range of our common
ancestors probably covered of much of Africa.

However, as we have learned, speciation likely occurs in small,
geographically isolated groups. And, at the dawn of the evolution of Hominidae
as a separate lineage from chimpanzee, Hominidae is found in East
Africa, but chimpanzee is not.

Rift valley

There exists a prominent geographical feature that separates the
ranges of the early humans and extant chimpanzees.

It is called the Rift Valley and it constitutes a long North-South
valley that separates East Africa from Central Africa (it is, to those
who appreciate plate tectonics, a proto-ocean forming as the East
African plate pulls away from central Africa).

Thus, on the west side of the Rift Valley (as it began to form as a
significant geographical barrier and as the chimpanzee-Hominid split
occurred) was found the chimpanzee ancestor living in a forested
environment that was similar to that in which the chimpanzee-Hominid
last common ancestor also had lived.

East side drying

On the eastern side of the Rift Valley environmental change occurred
which included a general drying trend.

This drying converted what was once forest into open savanna.

The dawn of Hominidae thus is thought to have coincided with both the
opening of the Rift Valley and an associated drying across the Hominidae
range, i.e., environmental change combined with geographical isolation.

By this reckoning, Hominidae diverged morphological from chimpanzee at
first and at least in part in the course of adaptation to a new, drier
environment.

Note that A. robustus was a sympatric (lived side by side)
contemporary of genus Homo, but which possessed a jaw and teeth
that were highly adapted to a rough and abrasive diet.

In fact, australopithecines in various forms survived long
after genus Homo was up and running. The last australopithecines
walked the earth approximately a million years ago.

All small-brained bipedals:

All species of genus Australopithecus are thought to have had
relatively small brain sizes (i.e., to body size), somewhere between
that seen with modern gorillas and modern chimpanzees.

Thus, no australopithecine would be considered large brained in
a genus Homo sense.

They also all appear to have been upright walkers.

Think about it, a mere 1 million years ago small brained, bipedal
hominids still walked the earth.

Australopithecines are not considered to be members of genus Homo
though they were hominids, and though genus Homo appears to have
evolved from genus Australopithecus (which, of course, makes genus Australopithecus
paraphyletic).

Man the
scavenger

Switch back to meat:

The evolution of genus Homo is thought to have been influenced
to a significant extent by the practice of basing a greater portion the
diet on the ingestion of meat than that obtained by either the great
apes or members of genus Australopithecus. Thus, we are Homo
the meat eaters.

However, were our ancestors who began this practice also the hunters
of the meat they began to consume in increasing amounts?

In fact, there is a high likelihood that concepts of man the hunter
are simply another example of man the arrogant.

Instead, it is more likely that early Homo obtained their meat
by stealing it from those much more adept at killing (leopards, for
example). That is, evidence that early Homo hunted is not nearly
as robust as evidence that early Homo scavenged.

Scenario:

One interpretation of the evidence goes like this:

Early Homo was omnivorous (eaters of both vegetables and
meat) and living in riparian habitats (rivers, which provided trees
for refuge from predators).

There they gathered plants which constituted the core of the early
Homo diet.

Especially during times of few (dry season, for example)
scavenging took on particular importance.

One source of meat was the bone marrow of large kills (brought
down by lions and saber toothed tigers), a source of meat available
only to them and the hyenas. Also potentially available were leopard
kills, the scavenging (actually stealing) of which was a role early Homo
alone may have filled.

"The earliest hominids probably scavenged and took small prey
with their hands, as chimpanzees and baboons do. Only their next step
was unique: they began to use tools to butcher large carcasses that
nonhuman primates cannot exploit. The difficulty of this leap (to the
use of tools to butcher) belies the charge that scavenging offers no
challenge that might select for human qualities. . . Scavenging is not
at all easy for a slow, small, dull-toothed primate. To locate
scavengeable carcasses before others did, we had to learn how to
interpret the diverse cues to the presence of a carcass in riparian
woodlands. They include the labored, low-level, early-morning, beeline
flight of a single vulture toward a kill; vultures perched in mid-canopy
rather than at the crown of a tree, where they nest; appendages of a
concealed leopard or of its kill dangling from a branch; and tufts of
ungulate hair or fresh claw marks at the base of a leopards favorite
feeding tree. At night, the loud 'laughing' of hyenas at a fresh kill,
the panicked braying of a zebra being attacked, the grunting of a
frightened wildebeest---all serve notice of where to find an abandoned
carcass when morning comes." (p. 94-95, Blumenschine and Cavallo,
1992)

Scavenging, brains, and hunting:

The bigger the primate brain, the more complex, varied, extensive, and
higher quality is the food supply potentially available to the
possessing organism.

Having access to a high quality food supply supplies two things:

it allows the support of a big brain, which, gram for gram, is the
most metabolically expensive tissue animals have

it maintains the species as generalists and therefore potentially
adaptable to new situations and environmental conditions

This may have in turn driven the evolution of an even larger brain
which ultimately set the stage for the later development of man the
hunter.

Hunting, of course, is an even more difficult strategy for obtaining a
varied diet, which in turn may have supplied one component of the
further selection pressure necessary to drive additional brain
expansion.

Perhaps of greatest significance, however, was H. habilis's
significantly increased reliance on the use of tools for eating and
survival.

"H. habilis was nearer to modern humans than A.
africanus is with respect to size and form of the brain, size and
morphology of the teeth, and the adjustment to an erect posture, as well
as in many other features. . . (also) H. habilis, it seems,
became obligate stone toolmakers and users, whereas the late
robust and hyper-robust australopithecines might have been only facultative
toolmakers and users (able to make and use stone tools, but equally
able to survive without them). . . The robust and hyper-robust
australopithecines (may have) prepared their food predominantly in their
mouths, chewing it between their greatly expanded upper and lower cheek
teeth, whereas H. habilis prepared its food with hand-held stone
tools prior to eating." (p. 158, 159 Tobias, 1992)

Anatomically modern H. sapiens began replacing the various
archaic forms approximately 100,000 years ago.

All of humanity is descended from these anatomically modern H.
sapiens.

Cultural evolution:

Anatomically modern H. sapiens have the duel distinction of:

having raised cultural evolution, the practice of passing
non-genetic information from parent to child, to higher levels than
any other animal could dream of

of using that skill to drive (largely not only without abatement
but to an increasing extent) her entire home planet to the brink of
ecological/environmental collapse.

Clearly humans have become very good at what we do. Perhaps we have
gotten a little too good?

Man's future:

This is not a course on anthropology so we will not consider
humanity's achievements beyond this except as they pertain to biological
sciences.

We will also avoid speculating on mankind's biological future except
to leave you pondering these important considerations:

Only with a knowledge of evolutionary biology can one successfully
subvert evolution.

The one indisputable rule of evolution is that all species go
extinct.

Profound extinction of lineages usually occurs as a consequence of
failure to adapt to changing environments (especially when those
changes are abrupt).

Consistently, seemingly omnipotent human civilizations of the past
have had a nasty tendency of collapsing due to self inflicted (or
perpetuated) environmental change. Indeed, our entire world-wide
civilization is just as dependent as it ever has been on the
existence of a few inches of top soil and the fact that more often
than not the rains come at appropriate times.

Modern man, at an unprecedented rate, is changing her environment
for the worst. Are you prepared to survive a changing environment?
Are any of us? Could you survive even a month in the absence of a
local food market and a few gallons of gasoline refined from oil
mined half-way around the world? In a world of so many, could any of
us?

The lay public and even scientists tend to use the term ancestor in a
very fast and loose manner when describing exciting fossils. Strictly,
an ancestor is an individual which you and or I can trace an unbroken
blood line back to. Thus, our great-grandparents, unless we were
adopted, were our ancestors.

All of humanity can trace their ancestry back to common individuals.
The last common ancestor of all of humanity was in all likelihood also
human. However, if one finds an ancient human fossil, does that imply
that that person is an ancestor to all of humanity? Not necessarily
(and, in fact, not likely). However, we might still describe that
individual as an ancestor to humanity because she was of the proper
species and lived at approximately the same time as we expect of our
true common ancestor.

As we go farther back in time, things start to become not only more
and more vague as to what exactly a true ancestor must have looked like,
but even what species a true ancestor must have been (or morphology must
have had). Thus, if you think of evolution has producing a bush of
diverging lineages, a true ancestor is found on a branch that is
directly between you and the center of the bush. Everything else is not
an ancestor in the strictest sense.

However, a separate species that was closely related to the true
ancestor might still be referred to as an ancestor. That is, with only a
slight divergence from the true lineage, combined with ultimate
extinction (since divergence from the chimpanzee ancestor, all members
on our branch but one have gone extinct) leaves it very difficult to
say, even down to the level of the species, whether a given individual
or group is in reality an ancestor (though its easier to argue that a
particularly divergent lineage is not). However, at this point
operationally it really stops making all that much of a difference.
Thus, an Australopithecus afarensis-like species was probably a
direct human ancestor, though A. afarensis itself may not have
been that ancestor, and, with high probability, any individual fossil
probably really isn't a true, direct ancestor either.

A last common ancestor basically the often hypothetical organism
which was the last species to exist prior to the divergence of that
species into the two or more distinct species being compared. Since, as
noted above, it is often very difficult to assign clear ancestral status
to a fossil, the last common ancestor is often not associated with
any given fossil with high certainty. Nevertheless, for any two divergent
lines, some last common ancestor must have existed at some time in
the past (assuming the existence of some universal ancestor). Often
through methods of molecular evolution the time of the divergence and
therefore that date at which the last common ancestor lived can be
estimated with reasonable precision.

Earliest common ancestor is probably a term I've coined here. I use
it to describe the first species of a lineage. That is, the base of the
tree---the root of the clade. Thus, the earliest common ancestor of
Hominidae is the first species in the Hominid lineage following divergence
from the chimpanzee lineage (i.e., the first species post-last common
ancestor). The earliest common ancestor is the root species of a
monophyletic tree.

Culture is information which is passed on from parent to offspring
(more generally, from the older to the younger) through non-genetic means.
For example, the ability to make stone tools or speak a specific language.
Or, in less anthrocentric terms, the ability to hunt or make a den, etc.

Cultural evolution is change in culture over time. Cultural
evolution can be very powerful to an intelligent generalist because the
means by which variation occurs often is directed (one tries some new thing
that one hopes will work) and the rate at which failed experiments may be
culled is often very rapid, while simultaneously not necessarily leading to
the extinction of the carrier of the variant. Thus, cultural evolution
occurs by a series of accidental or purposeful trials, errors, and culling,
and may be transmitted between individuals lacking a close blood relation.

What description best describes the phylogenetic grouping consisting of
(i) the gorillas, the chimpanzees, humans, and the gibbons; (ii) all of
the ancestors of these extant animals going back to their last common
ancestor; and (iii) all of the extinct lineages stemming from this last
common ancestral species? (circle best answer)

Pongidae

a monophyletic grouping

Hominoidea

a paraphyletic grouping

all of the above

none of the above

According to the hypothesis concerning the Rift Valley, where did
Hominids diverge from the chimpanzee-human most recent common ancestor?

According to the "man the scavenger" hypothesis, selection for
large brains (i.e., greater than those of the chimpanzee) initially
occurred within the context of a broadening of the hominid diet to include
scavenged meat. If so, which of the following either scavenged, or evolved
in the course of scavenging for meat?

Australopithecus afarensis

Australopithecus boisei

Australopithecus africanus

Australopithecus robustus

Homo habilis

Homo erectus

Homo sapiens archaic

Of extant apes, with which do humans show the greatest evolutionary
divergence?

Practice question answers

vi, Homo erectus

A hominid is a monophyletic subset of the hominoids, one
whose members exhibited, among other things, a bipedal gate. The hominoids
is a monophyletic clade consisting of all of the apes and the hominids.

iv, a paraphyletic grouping. That is, orangutans are
missing from the grouping.

December 7, 2000 BBC: CHRONOLOGY - Where did modern humans come from?
Despite all the information that scientists have been able to gather, one fact is especially true. The whole human gene pool has less diversity in it than the gene pool of a single troop of gorillas or chimpanzees. Humans are a very variable species in appearance, but there is more variation between a group of people in the same area than between people from opposite sides of the globe. The majority opinion today is that we share a recent common African origin, and that a lot of the differences between us really are of recent origin too.